The aim of the barrier principle is to contain sources of radiation within nuclear installations at multiple levels. Structured like the layers of an onion, independent barriers ensure that the risk arising from radiation sources is minimised for both people and the environment.

Within Swiss nuclear power plants, the barrier principle encompasses a sequence of three physical barriers to contain the radioactive substances present in the nuclear fuel.

Barrier 1: Fuel elements

Barrier 2: Primary circuit

Barrier 3: Containment